In 3D seismic prospecting, an areal array of seismic sources and receivers are positioned over an area of the earth's surface and seismic data are collected in the form of seismic traces generated by the receivers in response to acoustic waves. This is in contrast to two dimensional seismic prospecting wherein a linear array rather than an areal array of sources and receivers is utilized. In 3D as well as in two dimensional seismic prospecting it is desirable to “stack” a number of traces (commonly called a common midpoint bin or gather) which correspond to a number of source-receiver pairs which share a common midpoint position.
FIG. 1A illustrates a midpoint 105 positioned between a seismic source 101 and a seismic receiver 103. For any seismic source 101 with a receiver 103 there is a midpoint 105 that is the seismic survey position where the received signal data will be positioned for data processing purposes. For compressional seismic wave processing the midpoints are positioned a distance L/2 in the source to receiver direction for a source to receiver offset distance of L. Other waveforms or seismic acquisition and processing considerations may dictate ‘midpoints’ at varying locations, so the midpoints for the purposes of disclosure are for illustrative purposes.
In planning, collecting and processing a 3D seismic data, it is desirable to position the sources and receivers to optimize various conditions with respect to fold, offset and azimuth. FIG. 1B shows the areal positions of a simple 3D acquisition geometry. A source 101 is positioned relative to seismic receivers 103 and the midpoints 105 are situated between the source 101 and receivers 103. The midpoints 105 are where the data traces are positioned relative to all the data traces of the survey for purpose of binning the data traces. FIG. 1B illustrates a four-sensor receiver array. In modern practice, receiver arrays consist of hundreds or thousand of receivers. Receiver arrays are referred to as “swaths” or “patches.” FIG. 1 may be considered a “narrow-swath” because the azimuthal variation of source to receivers is relatively small when compared with FIG. 1C.
FIG. 1C illustrates a “wide-swath” receiver array acquisition geometry. It is termed wide-swath because the variation in acquisition of source to receiver azimuths for this geometry is much greater than illustrated in FIG. 1B. The various geometries of source to receiver layouts or patches that comprise wide or narrow swath acquisition schemes varies greatly and is well known to practitioners in the art.
As is known in the acquisition and processing art, the survey area localized within an entire seismic survey designated for the collection of common midpoint positions from multiple source-receiver pairs is termed a common midpoint “bin.” The data processing steps of organizing traces in bins is termed “binning.” A bin may contain many traces from source-receiver pairs. The number of traces that goes into a bin is termed the bin's fold. The common midpoint gather (CMP) used herein for purposes of an exemplary seismic data gather, although it is only one of many. Common depth point (CDP) gathers, common reflection point gathers, common image point gathers and common conversion point gathers are all examples of seismic gathers. For the CMP, the term “source-receiver pair” refers to a source position and receiver position located on opposite sides of a midpoint and spaced substantially equidistantly from the midpoint.
FIG. 2 illustrates the concept of how a midpoint bin is formed. A plurality of source-receiver pairs in a seismic survey will have midpoints that group in the same vicinity of the survey. In FIG. 2, sources 101A, 101B and 101C with their respective receivers 103A, 103B and 103C, contain midpoints that fall relatively close together, in a selected localized area, to form a midpoint bin. The midpoints that form this example midpoint bin are 105A, 105B and 105C.
Source-receivers pairs from midpoint bins are further processed by correcting statics problems and adjusting for velocity effects prior to stacking. Stacking of seismic traces corresponding to such source-receiver pairs involves summing of the traces after so as to enhance important reflection events in the traces and remove spurious noise which can obscure the reflection events. In other words, stacking enhances the signal to noise ratio.
With respect to fold, it is desirable to have an adequate fold for each common midpoint bin in order to give an acceptable signal to noise ratio in the resulting stacked trace. It may be desirable to have uniformity of fold among a maximum number of common midpoint bins for a particular areal array. This results in a more uniform signal to noise ratio for the various stacked traces and better uniformity across the survey. With such a uniform signal to noise ratio among stacked traces, variations of amplitude from trace to trace will be related to the strength of reflection events and not the difference in the number of traces being summed. This makes the seismic survey data a more accurate product.
Certain parameters which characterize a group of 3D source-receiver pairs corresponding to a particular common midpoint bin include fold, offset and azimuth. Fold refers to the number of source-receiver pairs sharing a common midpoint for which traces are stacked. For example, if there are 16 source-receiver pairs for a particular stack, there is “16 fold” for the midpoint. Offset is simply the distance between the source and receiver of a particular source-receiver pair. Azimuth is the angular orientation of the source-receiver pair. More precisely, the azimuth angle for a particular source-receiver pair is the angle defined between the line along which the source-receiver pair lies and an arbitrarily selected direction such as true east or north.
With respect to offset, it is desirable to have a variation of offsets for the source-receiver pairs corresponding to a particular common midpoint. The different offset values are utilized to derive velocity analysis corrections for the traces being stacked. Velocity analysis corrections are applied to seismic data to position reflecting events to their proper position in time. An “average stacking velocity” is derived from the velocity analysis corrections. Such a stacking velocity is used to correct for normal moveout among the traces. Maximizing the distribution of offset values serves to enhance the accuracy of the derived stacking velocity and thus also the accuracy of the resulting normal movement correction. Maximizing the distribution also serves to enhance the accuracy of Amplitude Variation with Offset (AVO) analysis which can be useful in determining rock and fluid properties.
With respect to azimuth, it desirable to have a maximum variation in azimuth angles among the source-receiver pairs corresponding to a particular common midpoint. By having many different azimuth angles, the accuracy of 3D statics solutions and velocity analysis is enhanced. Statics are corrections applied to seismic data to correct for low velocities (weathering velocities) of seismic waves encountered in unconsolidated sediments near the earth's surface.
U.S. Pat. No. 4,933,912 to Gallagher discloses a 3-D seismic prospecting method which employs an areal array of sources and receivers by which seismic traces are generated. The areal array is segregated into a plurality of shells and angularly separated sections from which a preselected number n1 of source-receiver pairs are selected for a particular common midpoint. By means of the shells and sections, the source-receiver pairs so selected have associated therewith a wide range of offsets and azimuth angles for the preselected fold n1. The seismic traces corresponding to the selected source-receiver pairs are summed to give a stacked trace corresponding to the common midpoint.
U.S. Pat. No. 5,963,879 to Woodward et al. discloses a method wherein three dimensional seismic survey data are acquired and processed using a hexagonal sampling grid. The seismic data are grouped into hexagonal bins defined by the hexagonal grid instead of into rectangular bins defined by a rectangular grid. Method and apparatus which bin the data, although described for square and regular hexagonal grids, are also applicable to rectangles and hexagons of any shape. Because hexagonal binning requires fewer grid points than rectangular binning, survey source or receiver interval may be increased, which may help reduce survey cost.
U.S. Pat. No. 6,026,059 describes processes for providing a data set useful for performing analysis of reflection attribute variation among traces in a window of three-dimensional seismic data, wherein the traces have a reflection point assigned thereto and wherein the traces represent recordings from shot-receiver pairs having various azimuth angles. According to one embodiment, the process comprises: assigning an offset value to a plurality of traces; fitting a substantially conical surface having a major and a minor axis to the data of the traces within the window, wherein: the major axis of the cone represents the azimuth direction having the lowest variation in the reflection attribute, and the minor axis of the cone represents the azimuth direction having the highest variation in the reflection attribute; assigning a coordinate set to the surface, wherein the coordinate set is related to a survey geometry of the data; and comparing the reflection attribute variation as a function of offset and azimuth.
U.S. Pat. No. 6,625,543 discloses a method wherein input seismic data are re-gridded to an arbitrary output grid by output-based azimuth moveout. An input seismic data set corresponding to an input grid is used to generate an equivalent output seismic data set corresponding to an output grid different from the input grid. Preferably, the output grid is divided into blocks, and each output grid block is assigned to one of a plurality of independent parallel processors. For each output trace corresponding to an output location, the contributions of plural input traces to the output trace are computed according to an azimuth moveout operator. The contributions are then summed into the output trace.